PSS: Allow backend or state_store config to be passed via BackendOpts into method for initialising the operations backend for a command

[SarahFrench]

Context: #37321 (comment)
(I figured that the commit in this PR made sense to introduce into main on its own, separate to implementing further logic for PSS in the init command.)

This PR connects the code that can parse state_store configuration with new logic introduced in #37321 that allows Terraform to create an operations backend that uses PSS. Prior to this PR the logic in that PR couldn't be invoked, as BackendOpts.StateStoreConfig and BackendOpts.ProviderFactory are never set based on what's in the config.

Now the logic prepares different BackendOpts depending on the situation:

• If PSS is in use: include state_store config and the provider factory needed for PSS in the BackendOpts value.
• If a configured backend is in use: pass through backend config in the BackendOpts value.
• If there's no config: pass through a minimal BackendOpts, as there is no config to pass through or other related fields.

Target Release

1.14.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[radeksimko]

LGTM except for the one in-line comment

[radeksimko]

I think we should be either checking for nil here or implement a safe Len() method on FlagNameValueSlice which checks for nil before de-referencing the pointer.